1. Field of the Invention
The invention relates generally to wellbore packer assemblies and, in particular aspects, to the design of slip devices that are carried upon such packer assemblies.
2. Description of the Related Art
Traditional packers are comprised of an elastomeric sealing element and at least one mechanically set slip. Typically, a setting tool is run in with the packer to set it. The setting can be accomplished hydraulically due to relative movement created by the setting tool when subjected to applied pressure. This relative movement causes the slips to ride up on cones and extend into biting engagement with the surrounding tubular. At the same time, the sealing element is compressed into sealing contact with the surrounding tubular. The set can be held by a body lock ring, which would prevent the reversal of the relative movement that caused the packer to be set in the first instance.
As an alternative to applying pressure through the tubing to the setting tool to cause the packer to set, another alternative was to run the packer in on wire line with a known electrically-operated setting tool, such as an “E-4”-style setting tool that is available commercially from Baker Oil Tools of Houston, Tex. In setting the packer device, a signal fires the E-4 causing the requisite relative movement for setting. If the packer device is of a retrievable type, a retrieving tool could later be run into the set packer and release the grip of the lock ring and allow movement of the slips back down their respective cones and a stretching out of the sealing element so that the packer device can be removed from the well.
Some packer designs seek to create an engagement of packer element slips or wickers by direct radial expansion of an expansion mandrel that carries slips or wickers. Examples of such expandable packer designs are found in a parent application to this one, U.S. Patent Publication No. U.S. 2005/0028989 A1. This Publication describes packer devices that are set by radially expanding an outer expansion mandrel in response to fluid pressure from the flowbore. U.S. Patent Publication No. U.S. 2005/0028989 A1 is owned by the assignee of the present invention and is hereby incorporated by reference.
A problem arises when a separate slip mandrel is placed to radially surround the expansion mandrel. A slip mandrel is generally a unitary tubular component having a substantially smooth radially inner surface and an outer radial surface with engagement wickers integrally formed thereupon. The addition of this unitary component will substantially increase the setting force required to expand the expansion mandrel and urge the wickers into biting engagement with the surrounding tubular.
Placing axial slots in the slip mandrel to make it easier to expand is problematic. Slotting the slip mandrel will necessarily weaken it and possibly render it vulnerable to rupturing during expansion of the expansion mandrel.
U.S. Pat. No. 6,378,606 issued to Swor et al. shows a barrel slip body having axial slots that divide the slip body into a series of separate barrel slip anchors. The slots extend along almost the entire length of the barrel slip body. During setting, wedge cones are axially moved to cam the slip portions of the barrel slip radially outwardly and into a set position. During axial setting, the slip portions move outwardly substantially uniformly along their lengths.
The type of barrel slip described in the Swor patent would be unsuitable for use with a radially expandable packer that uses a radially expandable expansion mandrel to urge the slip mandrel into setting. The forces involved in setting the slip wickers are substantially different with the radially expandable packer than with an axially set one. With the radially expandable packer, the expansion mandrel expands radially outwardly in a manner that is substantially uniform along the axial length of the slip mandrel. However, there may be portions at the axial ends and in the axial center of the expansion mandrel that are intended to expand to a greater degree than the other portions of the expansion mandrel. Thus, a more suitable slip mandrel design is needed for use with radially expandable packer devices.
The present invention addresses the problems of the prior art.